Each channel manager can address (access data in) one stick at a time.
Two (Dual) channel managers can access two modules simultaneously.
Two equal RAM modules of identical specifications, will allow the memory managers to run at optimum performance.
Depending on what your motherboard's manual has to say, you may be able to use differing specs and sizes, 1,2,3, or 4 modules and so on.
But often it will perform at the specs of lowest common denominator between differing modules.
And at last, portions of the differing RAM may be accessed in single channel mode.
At worst case.. accessing even the worst performing RAM configurations is still on the order of a thousand times faster than going to the hard drive.